EDGE-RUNNER MILLS 379
smaller than -J- in. diameter, as they are liable to be easily clogged
and require frequent cleaning. If a pan mill is used to grind a
material to powder, the material from the mill should be screened
and the coarser material returned to the mill to be re-ground.
Although all the material thus returned is capable of passing through
the perforations in the mill, it will, if delivered to a solid part of
the pan, be reduced to much smaller particles before being dis-
charged through the perforations. It is by no means unusual to
employ screens of 24-mesh, in conjunction with |-in. perforations
in the pan, with completely satisfactory results.
The holes in the pan should taper so as to be larger on the
under surface of the pan than on the upper one ; this largely
prevents the holes from being clogged.
Edge-runner mills used for dry grinding must be very strongly
built ; the runners should be as heavy as possible, and are best if
constructed of iron and fitted with renewable rims or tyres of steel
or special hardened metal. The employment of these removable
tyres has many advantages, one of the most important being the
cheapness with which the working surface of the runner can be
kept reasonably level.
Some firms prefer the runners to be made of stone, and where
it is necessary to avoid the use of iron this material is excellent.
For most purposes, however, it is better to fix on them a renewable
type of manganese steel or chilled iron, as unprotected stone wears
away rapidly. An excellent arrangement when transport charges
are heavy is to make a skeleton runner of iron and to fill it with
concrete when the runners have reached their destination.
The runners must be so constructed as to allow of their vertical
movement through a distance of at least 6 in., so that in the event
of a large mass, too hard to be immediately crushed, getting into
the mill, the runners may pass over it. If this cannot be done,
an excessive strain is placed on the driving gear and an accident
may result. One excellent method applicable to mills with revolving
pans is to suspend each roller by means of two chains, but a more
usual one is to provide slots in the standards carrying the ends
of the runner axles.
From the point of view of durability, the best pan mill is the
one which has the maximum output for the smallest pan-area ;
hence it is necessary that the scrapers should be fixed very care-
fully so as to work with the greatest efficiency. They should be
arranged so as to mix the material thoroughly and to pass it as
often and as completely as possible beneath the runners. Each
scraper should be capable of separate adjustment, both horizontally,
so as to ensure that it moves the material in the desired direction,
and vertically, so that it may be lowered as the metal wears
away. Some firms on the Continent use hanging scrapers which
rest on the material by their own weight and can move vertically
without being adjusted. If the scrapers are fixed to arms pivoted
on a shaft, at the back of the machine, the front of the pan may